Re: [PATCH 7/8] membarrier: Remove arm (32) support for SYNC_CORE
From: "Russell King (Oracle)" <linux@armlinux.org.uk>
Date: 2021-06-16 15:23:42
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On Wed, Jun 16, 2021 at 04:04:56PM +0100, Catalin Marinas wrote:
On Wed, Jun 16, 2021 at 02:22:27PM +0100, Russell King wrote:quoted
If it's a problem, then it needs fixing. sys_cacheflush() is used to implement GCC's __builtin___clear_cache(). I'm not sure who added this to gcc.I'm surprised that it works. I guess it's just luck that the thread doing the code writing doesn't migrate before the sys_cacheflush() call.
Maybe the platforms that use ARM MPCore avoid the issue somehow (maybe by not using self-modifying code?)
quoted
Likely only in places where we care about I/D coherency - as the data cache is required to be PIPT on these SMP platforms.We had similar issue with the cache maintenance for DMA. The hack we employed (in cache.S) is relying on the MESI protocol internals and forcing a read/write for ownership before the D-cache maintenance. Luckily ARM11MPCore doesn't do speculative data loads to trigger some migration back.
That's very similar to the hack that was originally implemented for MPCore DMA - see the DMA_CACHE_RWFO configuration option. An interesting point here is that cache_ops_need_broadcast() reads MMFR3 bits 12..15, which in the MPCore TRM has nothing to with cache operation broadcasting - but luckily is documented as containing zero. So, cache_ops_need_broadcast() returns correctly (true) here.
The simpler fix for flush_icache_range() is to disable preemption, read a word in a cacheline to force any dirty lines on another CPU to be evicted and then issue the D-cache maintenance (for those cache lines which are still dirty on the current CPU).
Is just reading sufficient? If so, why do we do a read-then-write in the MPCore DMA cache ops? Don't we need the write to force exclusive ownership? If we don't have exclusive ownership of the dirty line, how can we be sure to write it out of the caches? -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!